This invention pertains generally to catoptric lens arrangements and particularly to such types of arrangements having lens elements with focal points spaced from the lens axis.
This application is a continuation-in-part of our copending application entitled "Mirror and Method of Making Same," Ser. No. 268,271, filed July 3, 1972 and assigned to the same assignee as this application.
It is now known in the art that so-called "confocal" catoptric lens arrangements, i.e. arrangements incorporating reflecting surfaces corresponding to the curved surface generated by nutating selected quadratic conic sections about a lens axis, combine many desirable qualities. Reflecting surfaces so generated characteristically possess image points on a circle, or an arc of a circle, centered on an axis rather than a single point as in the usual case. Thus, by judiciously selecting particular conic sections for the generatrices of the reflecting surfaces of the elements of a catoptric lens arrangement, it is possible to design such an arrangement to be diffraction-limited with an extremely large aperture. It follows, then, that the f-number of such a lens arrangement may be far less than 1. Such a characteristic, in turn, means that radiant energy from a point source may, if desired, be focused within a very small circle of confusion; as a matter of fact, focusing may take place within a circle of confusion with a diameter in the same order as the wavelength of radiant energy. Alternatively, if desired, an almost perfectly collimated beam of radiant energy may be formed from radiant energy from a point source, because the diffraction effects suffered by such a catoptric lens arrangement are very small.
It is evident that confocal catoptric lens arrangements are particularly well suited for applications in which conventional refractive lens arrangements are, for one reason or another, not satisfactory. For example, when the radiant energy to be focused or collimated is concentrated in an intense beam, as in the beam from a high-powered laser, a sufficient amount of such energy incident on a refractive lens arrangement is absorbed by the material from which the lens elements are fabricated (some type of glass, mica or other solid material ordinarily deemed to be totally transparent) thereby causing undue heating which distorts, or even destroys, the lens elements.
Although a catoptric lens arrangement is not as susceptible to damage from overheating because energy passing through such an arrangement is almost completely reflected by the mirror surfaces of the lens elements, there is, however, a slight amount of energy absorbed by each lens element to cause heating. Therefore, especially when it is necessary to combine beams from more than a single high powered laser, even catoptric lens arrangements may be unduly heated. There simply is no known way of making the reflecting surface of lens elements in a catoptric lens arrangement to provide reflecting surfaces which are certain to withstand the enormous concentrations of energy resulting from the use of several high power lasers. Further, with any catoptric lens arrangement not using confocality principles, appreciable aperture blockage must be tolerated in order to arrange the lens elements in proper relative position with respect to each other.
It has been proposed to carry out thermal nuclear fusion by combining the beams from a number of high powered lasers in such a manner that the energy in each beam is focused on a small target. It is possible in such a system to obtain a flux density at the target which is sufficiently high to initiate the fusion reaction. That is, a flux density in the order of 10.sup.14 to 10.sup.16 watts per square centimeter may be attained. When energy with a flux density of such intensity is attained, a concomitant light pressure (in the order of the pressure required to contain the expanding plasma resulting from a nuclear reaction) is generated. Obviously, however, successful containment of an expanding plasma from a real specimen undergoing fusion requires that the light pressure be applied over a continuous finite area. In other words, any lens arrangement suited to the purpose must be capable of forming an "optical bottle."
In the design of optical radars it would be highly desirable to combine beams from several lasers into a composite beam, thereby to increase the effective range of the system. Again, in such an application conventional lens arrangements are inadequate for the basic reason that it is almost impossible to collimate energy from more than one source into a single beam. Using conventional lens elements, which have their focal points on a lens axis, in practice only one laser beam may be collimated by any particular known lens arrangement.
It has been proposed that numerically controlled lathes be used to turn mirror surfaces for catoptric arrangements such as those mentioned above. If such a lathe is used to turn an optically smooth mirror surface (meaning a surface which, after lapping, conforms to a desired shape within a fraction of a wavelength of the radiant energy to be reflected) the precision with which the turning operation must be carried out makes it extremely difficult to machine any but relatively small and simple mirror surfaces. Some of the critical parameters are: (1) machine accuracy; (2) precision of control of turning speed and feed; and (3) hardness of the material on which the mirror surface is to be formed.
Machine accuracy encompasses the usual considerations required to be met by any precision lathe used for "micromachining." First of all, then, extreme care in the mechanical design and construction of the lathe itself must be taken to reduce errors such as those resulting from bearing runout of the spindle of the lathe or backlash in the lead screw of the lathe. Because control of the relative positions of the cutting tool and the workpiece is determined by successive outputs of a controlling digital computer, successive ones of such positions are separated by discrete amounts. Obviously, then, if the final surface must correspond to a given curve within a tolerance, say in the order of one micron, the incremental changes in the output of the controlling digital computer must be less than tolerance permitted for error in the final surface. Such a requirement in turn means that the programming of the controlling digital computer is detailed or the memory of such computer is very large to contain a very large number of positioned control signals.
Machining a mirror surface on a numerically controlled lathe requires, for satisfactory results, that the relative linear speed between the cutting tool and the workpiece be held almost constant during each machining pass. That is, the "surface feet per minute" of material removed from the workpiece must be maintained at a constant value to avoid wave-like distortions in the finished mirror surface. While"surface feet per minute" may be maintained at a constant value by correspondingly maintaining spindle speed when turning a cylindrical mirror surface, a different situation obtains when any other shape of mirror surface is desired. In any such situation, it is necessary that, during any given pass, the spindle speed be varied in a manner dependent upon the particular curve being machined. This requirement, in turn, means that the controlling digital computer should be adapted to control spindle speed in a different way for each differently shaped mirror surface.
Whenever a mirror surface is to be turned on a numerically controlled lathe, it is particularly important that the sharpness of the cutting tool not change appreciably during any pass. It has been found, however, that even diamond-tipped cutting tools are susceptible to excessive wear if the material of the workpiece is other than a relatively soft material, as gold, silver or copper. Unfortunately, however, satisfactory turning of such materials requires that the surface feet per minute be maintained within extremely close tolerances during each pass to avoid wave-like distortions in the final mirror surface. Therefore, only relatively small mirror surfaces of relatively simple shapes may be produced even with the most sophisticated and accurate known numerically controlled lathes.